This invention relates to an optical scanner for character or bar code recognition systems and is directed more particularly to a multiple head optical scanner for reading a column of video information.
Many types of document handling apparatus require recognition systems for reading characters or bar codes on the documents. Some types of mail sorting equipment, for example, use optical character recognition (OCR) systems for sorting mail according to zip codes written on the envelopes. Other types of mail sorting equipment use bar code recognition (BCR) systems for sorting mail according to a bar code printed on the envelopes.
In optical character or bar code recognition systems, light reflected from a source document is processed so as to identify the characters or bar codes on that document. Such recognition systems usually comprise an optical scanner, signal processing circuitry, and recognition logic circuitry. The optical scanner or video lift is used to detect the magnitude of the light flux reflected from various points on the source document and to convert that information to electrical signals. The signal processing circuitry converts these electrical signals to digital signals representing the differences in contrast between the characters or bar codes on the source document and the background of the source document. These digital signals are processed by the recognition logic circuitry which identifies the characters or bar codes read from that document.
Currently, two types of video lifts are widely used in recognition systems for document handling apparatus. The first type of video lift is a flying spot scanner which uses a movable scanning beam to read a stationary source document. Many flying spot scanners use raster scan techniques and operate as a television camera tube to scan each row of a document in a point-by-point manner. The scanner produces an electrical signal for each point scanned. The series of electrical signals are processed and the video information from the document is identified. Other flying spot scanners use contour tracing techniques to follow the contour of each character. The horizontal and vertical deflections of the scanning beam are processed to identify each character scanned.
One problem with using conventional flying spot scanners in OCR or BCR systems for mail sorting equipment is that the source document remains stationary when that document is being read. Consequently, any document transport system must intermittently move a document to a read station before reading, stop the document during reading, and remove that document from that station after reading. This intermittent moving of a document not only increases the complexity of the transport system, but also limits the overall speed of the system. Although some document handling apparatus have been designed to use flying spot scanners to read moving documents, such apparatus have been overly complex or have required multiple read stations. See, for example, U.S. Pat. No. 3,846,753 by Arie A. Spanjersberg entitled "Automatic Address Detection System."
The second type of video lift used in document handling apparatus is a data lift which uses a stationary read head to scan a moving source document. Such read heads include a linear array of photosensitive cells aligned orthogonally to the motion of the document and a lens system for projecting the image at a scanning window onto the linear array. The cells of the array are scanned at a rate which is relatively high compared to the motion of the document through the read station. As a result, the array scans each character or bar code moving past the scanning window a multitude of times and generates a series of electrical signals. These signals may then be processed to identify the characters or bar codes on the source document.
One problem with data lifts having linear arrays is that current fabrication techniques limit the practical size of these linear arrays having acceptable resolution. Although some relatively large arrays having acceptable resolution have been fabricated, the cost of producing these arrays has been excessive. Consequently, the scanning height of data lifts using economical linear arrays is limited.
In many types of document handling apparatus, the location or position of the characters to be read cannot be accurately determined. Mail sorting machines, for example, read the zip code at the bottom of the mailing address. Due to varying sizes of envelopes, differing placements of the addresses, and other factors, the height of the area or window to be scanned by the data lift often exceeds the specifications of data lifts having linear arrays. As a result, data lifts using linear arrays have not been completely satisfactory for use on mail sorting machines.
In order to overcome the height restriction of data lifts, several attempts have been made to increase the scanning height of such data lifts. One such attempt is shown in the U.S. patent of R. J. Rabinow, et al, U.S. Pat. No. 3,538,499, entitled "Optical Reading Machine." This patent discloses an optical scanner comprising a linear array, several sets of light pipes, and several lens and shutter assemblies. The number of lens and shutter assemblies corresponds to the number of sets of light pipes and the number of light pipes in each set corresponds to the number of elements in the photosensitive array. The lens assemblies are positioned to successively read parallel lines of print from the document. Each lens assembly focuses the light reflected from one line of print onto one end of one of the sets of light pipes. The other end of each respective light pipe is aligned with a corresponding element of the photosensitive arrays. Shutters are provided so only one lens assembly and its corresponding set of light pipes is activating the photosensitive array at a time.
As a document moves through the read station of the above optical scanner, the first shutter operates allowing the first lens assembly to direct light reflected from one line of print onto the first set of light pipes and, therefore, to the photosensitive array. After that line of print has been read, the first shutter closes while the second shutter opens, thereby allowing the second lens assembly, second set of light pipes, and the photosensitive array to read the next line of print. Successive lines of print are read in a similar manner.
Although the scanner described in the Rabinow et al patent does permit multiple lines of print to be read by a single read head, that scanner still requires the lines of print to be located in relatively constant positions. Thus, the Rabinow et al patent does not describe an optical scanner suitable for use with mail having varying locations of the addresses on the envelopes.
Other attempts to increase the scanning area of a read head use beam splitters to direct light reflected from a document onto various arrays. One such arrangement is shown in the U.S. patent of William E. Althauser et al, U.S. Pat. No. 4,129,853, entitled "Method and Apparatus for Producing Compensated Signals for Individual Light Sensors Arranged in a Predetermined Pattern." Another arrangement is shown in the IBM Technical Disclosure Bulletin of D. L. Janeway, III and A. L. Mix, Jr., Vol. 20, No. 1, June 1977, entitled "Multiple Diode Array Optics." In these publications, the optical scanner includes a plurality of linear photodiode arrays, a beam splitter for dividing a light beam among the diode arrays, and a single lens for directing the light reflected from the source document towards the beam splitter and for focusing that light onto the linear arrays. This arrangement allows the plurality of photodiode arrays to operate as if the scanner comprised a single photodiode array having a single lens.
One problem with the above scanner is that the arrays must be in an exceptionally critical alignment with the beam splitter so the light beam may be split among the arrays. Another problem with that scanner is that the arrays and the lens must be precisely adjusted so the lens will accurately focus the light onto the photodiode arrays. As a result of these critical alignments and precise adjustments, these scanners are not completely satisfactory for use on equipment having significant vibration, such as high speed mail sorting equipment.